The present disclosure relates generally to the manufacturing of semiconductor devices, and more particularly to a photolithography process in semiconductor manufacturing.
Since the inception of the semiconductor industry, photolithography has been used for forming the components of integrated circuits. Generally, light beams pass through a mask, which has been patterned with a magnified image of the relevant integrated circuits. The light beams are then focused by a projection lens on to a wafer, resulting in an image of the integrated circuits in the photoresist layer of the wafer.
Among other factors, the resolution of the image is related to the radiation wavelength and the numerical aperture of the optical system. Specifically, it is desirable to achieve a combination of a small wavelength and a large numerical aperture for printing dense circuits.
Enhancements are often needed to accommodate the increased density of integrated circuits. Some enhanced lithography techniques focused on reducing the radiation wavelength. Currently, state of the art lithography systems use 193 nm as the radiation wavelength for producing semiconductor devices that include more than one half billion transistors on each device.
However, it is impractical to continue reducing the radiation wavelength, as light beams with a wavelength smaller than 193 nm are absorbed by, rather than pass through, projection lenses that convey the light beams onto the wafer.
Therefore, to continue the advancement of semiconductor fabrication, it is desirable to further enhance the lithography by, for example, improving the numerical aperture of the optical system. One such enhanced lithography technique that achieves an improved numerical aperture of the optical system is immersion lithography. In immersion lithography (also known as wet lithography), water is inserted between the projection lens and the wafer (in contrast, air is permeated between the projection lens and the wafer in dry lithography). Since water has a refractive index of 1.4, the resulting numerical aperture of the optical system is increased by a factor of 1.4. Accordingly, image resolution may be significantly enhanced.
Although immersion lithography works well in increasing image resolution during semiconductor fabrication, multiple exposures are still often necessary for the required critical dimension (CD) uniformity and endcap performance with respect to certain semiconductor devices. The costs associated with enhanced lithography techniques like immersion lithography can therefore be undesirable, especially with multiple exposures.